Paper coating slip containing n-vinyl formamide

ABSTRACT

Paper coating slips whose additives comprise addition polymers or copolymers containing N-vinylformamide in (co)polymerized form, and their use.

[0001] The invention relates to novel paper coating slips. The inventionfurther relates to the use of paper coating slips and to papers coatedwith said slips.

[0002] Paper coating slips consist essentially of a pigment (usuallywhite), a polymeric binder, and additives which exert the desiredinfluence over Theological properties of the slip and surface propertiesof the coated paper. Such additives are frequently also referred to ascobinders. The function of the binder is to fix the pigments to thepaper and ensure cohesion within the coating obtained.

[0003] Slip coating gives base papers a smooth, uniformly white surface.The slips also enhance the printability of the paper.

[0004] The coating of paper with slips is nowadays well known; see, forexample, “The Essential Guide to Aqueous Coating of Paper and Board”, T.W. R. Dean (ed.), published by the Paper Industry Technical Association(PITA), 1997.

[0005] One of the most important objectives of the slip coating of paperis to increase the whiteness of the paper. At the same time, however,the surface of the paper must be sufficiently stable that it is notdamaged during printing and that the printed image is not impaired.

[0006] A host of different measures may be taken to increase thewhiteness of the coated paper. These measures include, for example, theuse of a base paper of very high whiteness, which in turn can beobtained by using very white starting materials. Another measure is toselect very white pigments for the paper coating slip.

[0007] These measures alone, however, are in the majority of casesunable to meet the modern-day requirements of users regarding thewhiteness of the paper. It is for this reason that products known aswhiteners (fluorescent or phosphorescent dyes) or optical brightenersare added to the coating slip. The brighteners are dyelike fluorescentdyes which absorb the shortwave ultraviolet light that is invisible tothe human eye and emit it as longer-wave blue light, giving the humaneye the impression of a higher whiteness, so that the whiteness isincreased.

[0008] The use of the optical brighteners, however, only results in thedesired outcome if the brighteners are present in an optimum structure,conformation, and distribution in the finished paper coating. To achievethis, polymeric compounds, which intensify the effect of the opticalbrightener and are referred to as activators or carriers, are added tothe slip. An important function of the cobinders—mentioned at theoutset—in colored coating slips is their brightener-activating effect.Suitable cobinders may include water-soluble polymers, e.g., polyvinylalcohol, carboxymethylcellulose, anionic or nonionic degraded starches,casein, soy protein, and water-soluble styrene-acrylate copolymers (see,for example, K. P. Kreutzer, Grundprozesse der Papiererzeugung 2:Grenzflächenvorgänge beim Einsatz chemischer Hilfsmittel, H. -G. Völkeland R. Grenz (editors), PTS Munich, 2000, PTS manuscript: PTS-GPE—SE2031-2).

[0009] However, it is not possible with every water-soluble polymer toactivate the optical brightener; for example, with certainpolysaccharides, e.g., dextran, or with anionic polyacrylamides, it isnot possible to activate the optical brightener. On the contrary, it haslong been known that water-soluble polymers containing cationic groups,especially amino or ammonium groups, such as polyamine/epichlorohydrinor polyamidoamine/epichlorohydrin resins, polyamines orpolyethyleneimines, for example, not only do not boost the whitener butin fact cause its deactivation, so that the brightening is extinguished(K. P. Kreutzer, loc. cit., page 8-22).

[0010] DE-A 197 27 503 discloses paper coating slips which comprisebinders containing N-vinylcarboxamide units. The activation of opticalbrighteners by additives, however, is not described.

[0011] It is an object of the present invention to provide paper coatingslips having improved properties or leading to an improvement in thecoated paper.

[0012] We have found that this object is achieved by the use of papercoating slips having improved properties, these slips including amongtheir additives addition polymers or copolymers containingN-vinylformamide (formula I) in (co)polymerized form.

[0013] In accordance with the invention it is possible as additives(cobinders) to use addition polymers or copolymers, referred tohereinbelow as (co)polymers, which consist of N-vinylformamide and,respectively, contain N-vinylformamide and also anionic, cationic and/ornonionic monomers in copolymerized form. Moreover, before their use in apaper coating slip of the invention, the polymers or copolymers may besubjected to a cleavage operation in which the carboxylic groups (formylgroups) are partially eliminated.

[0014] It is surprising that the addition of water-soluble (co)polymersof vinylformamide brings about extraordinarily great activation of theoptical brighteners in paper coating slips. Even more surprising andentirely unexpected, however, is that cationic copolymers ofvinylformamide—in contrast to what is stated by K. P. Kreutzer, loc.cit., page 8-22—are likewise suitable as carriers for opticalbrighteners and indeed boost their activity even more than the nonionicpolyvinylformamides. The activation of the whiteners in paper coatingslips by (co)polymers of vinylformamide, in accordance with theinvention, is much greater than the activity of other cobinders in thisrespect. In addition to their brightener-activating effect, the(co)polymers incorporated into the paper coating slips possess theproperty of raising the dry pick resistance and wet pick resistance ofthe coated paper, and do so to a greater extent than do other cobinders.Furthermore, it has surprisingly been found that papers coated with theslips of the invention produce a higher print gloss than papers coatedwith slips comprising prior art cobinders.

[0015] The preparation of the polymers of N-vinylformamide which may beused for the coating slips of the invention has been known for a longtime (see, for example, EP-B1 71 050, corresponding to U.S. Pat. No.4,421,602).

[0016] Cationic copolymers containing N-vinylformamide in copolymerizedform may also be used in accordance with the invention.

[0017] The preparation of cationic copolymers of N-vinylformamide and awater-soluble basic monomer, such as N-trialkylammoniumalkylacrylamides,N-trialkylammoniumalkylmethacrylamides and/or diallyldialkylammoniumsalts, for example, and their use as flocculants and dewatering aids forthe treatment of wastewaters and sludges, are described in EP-B1 464 043(corresponding to U.S. Pat. No. 5,225,088).

[0018] For example, as a water-soluble cationic monomer,diallyldimethylammonium chloride, diallyldiethylammonium methosulfate,N-(2-trimethylammonium)ethylacrylamide methosulfate orN-2-(ethyldimethylammonium)ethylmethacrylamide ethosulfate or mixturesthereof may be copolymerized with N-vinylformamide as described in EP-B1464 043, to give cationic copolymers suitable for use in the coloredpaper coating slips of the invention.

[0019] It is of course also possible, for the coating slips of theinvention, to use cleaved vinylformamides (see below) in which theliberated, polymer-bound amino functions form an ammonium formate withthe eliminated formic acid.

[0020] In addition, the preparation of copolymers of N-vinylformamideand monoethylenically unsaturated carboxylic acids having from 3 to 8carbon atoms and/or their alkali metal, alkaline earth metal or ammoniumsalts, such as acrylic acid or methacrylic acid, for example, and also,if desired, other ethylenically unsaturated, copolymerizable compounds,and their use as additives to the paper pulp for the purpose ofincreasing the dewatering rate and the retention in paper making, andalso the dry and wet strength of the paper, are known from DE-A1 42 41117 (corresponding to U.S. Pat. No. 5,630,907) and lead to anioniccopolymers which may likewise be used in accordance with the inventionfor coating slips.

[0021] Examples of suitable monomers of monoethylenically unsaturatedcarboxylic acids having from 3 to 8 carbon atoms, and of thewater-soluble salts of these monomers, include the following: acrylicacid, methacrylic acid, dimethylacrylic acid, ethacrylic acid, maleicacid, citraconic acid, methylenemalonic acid, allylacetic acid,vinylacetic acid, crotonic acid, fumaric acid, mesaconic acid, anditaconic acid. From this group of monomers it is preferred to useacrylic acid, methacrylic acid, maleic acid or else mixtures of saidcarboxylic acids, especially mixtures of acrylic acid and maleic acid orof acrylic acid and methacrylic acid. These monomers or mixtures thereofmay be used either in the form of the free carboxylic acids or in partlyor fully neutralized form for the copolymerization.

[0022] The weight ratio of vinylformamide to monoethylenicallyunsaturated carboxylic acid having from 3 to 8 carbon atoms and/oralkali metal, alkaline earth metal or ammonium salts thereof in themonomer mixture may range between 100:0 and 70:30, preferred weightratios being between 100:0 and 80:20, and particularly preferred weightratios being between 100:0 and 90:10.

[0023] Additionally, nonionic copolymers of N-vinylformamide withfurther water-soluble vinyl monomers may boost the activity of theoptical brightener in the paper coating slips of the invention. Furtherwater-soluble vinyl monomers suitable for this purpose includeN-vinylpyrrolidone and other N-vinyllactams, such as N-vinylcaprolactam,and also N-vinyl-N-alkylcarboxamides or N-vinylcarboxamides, such asN-vinylacetamide, N-vinyl-N-methylformamide, andN-vinyl-N-methylacetamide, for example. It is also possible to usemixtures of these.

[0024] A preferred monomer is N-vinylpyrrolidone.

[0025] The composition of the (co)polymers is generally as follows:

[0026] N-Vinylformamide:

[0027] 1-100 parts by weight, preferably 2-80, with particularpreference 5-80

[0028] Water-soluble cationic monomer:

[0029] 0-10 parts by weight, preferably 0.5-8, with particularpreference 1-5

[0030] Acrylic acid or methacrylic acid and/or their salts or mixturesthereof:

[0031] 0-30 parts by weight, preferably 1-20, with particular preference2-10

[0032] Further water-soluble vinyl monomer:

[0033] 0-90 parts by weight, preferably 0.5-80, with particularpreference 5-50

[0034] A method frequently used, but not the only one, for preparing theabovementioned (co)polymers is that of free-radical (co)polymerizationin a solvent or diluent.

[0035] The free-radical (co)polymerization of such monomers takes place,for example, in aqueous solution in the presence of polymerizationinitiators which break down into free radicals under polymerizationconditions. The (co)polymerization may be performed within a widetemperature range, where-appropriate at subatmospheric orsuperatmospheric pressure, generally at temperatures up to 100° C. ThepH of the reaction mixture is commonly adjusted so as to be within therange from 4 to 10.

[0036] The (co)polymerization may also, however, be conducted in otherways known per se to the skilled worker, for example, as a solution,precipitation, water-in-oil emulsion or inverse suspensionpolymerization. Solution polymerization is preferred.

[0037] The N-vinylformamide is (co)polymerized using free-radicalpolymerization initiators, examples being azo compounds that break downinto free radicals, such as 2,2′-azobis(isobutyronitrile),2,2′-azobis(2-amidinopropane) hydrochloride or4,4′-azobis(4′-cyanopentanoic acid).

[0038] Said compounds are usually used in the form of aqueous solutions,the lower concentration being determined by the amount of water that isacceptable in the *(co)polymerization and the upper concentration beingdetermined by the solubility of the respective compound in water. Ingeneral, the concentration is from 0.1 to 30% by weight, preferably from0.5 to 20% by weight, with particular preference from 1.0 to 10% byweight, based on the solution.

[0039] The amount of the initiators is generally from 0.1 to 10% byweight, preferably from 0.5 to 5% by weight, based on the monomers to be(co)polymerized. It is also possible to use two or more differentinitiators in the (co)polymerization.

[0040] Examples of solvents or diluents used include water, alcohols,such as methanol, ethanol, n- or iso-propanol, n- or iso-butanol, orketones, such as acetone, ethyl methyl ketone, diethyl ketone oriso-butyl methyl ketone.

[0041] If desired, the (co)polymerization may be conducted in thepresence of polymerization regulators, such as hydroxylammonium salts,chlorinated hydrocarbons, and thiocompounds, such as tert-butylmercaptan, 2-ethylhexyl thioglycolate, mercaptoethynol,mercaptopropyltrimethoxysilane, dodecyl mercaptan, and tert-dodecylmercaptan, or alkali metal hypophosphites. In the (co)polymerizationthese regulators may be used, for example, in amounts of from 0 to 0.8part by weight per 100 parts by weight of the monomers to be(co)polymerized, and have the effect of reducing the molar mass of theresultant (co)polymer.

[0042] In the case of emulsion polymerization, ionic and/or nonionicemulsifiers and/or protective colloids or stabilizers are used assurface-active compounds.

[0043] Depending on the polymerization conditions, the(co)polymerization produces (co)polymers of varying molecular weights,the molecular weight being characterized in EP-B1 71 050 and below usingthe Fikentscher K values (measured in 0.5% strength by weight aqueoussodium chloride solution at 25° C.). (Co)polymers having a high K value,of above 80, for example, are preferably prepared by (co)polymerizatingN-vinylformamide in water. (Co)polymers having a high K value with highmolecular weights are obtained, furthermore, by (co)polymerizing themonomers in the form of inverse suspension polymerization or by(co)polymerizing the monomers by the technique of water-in-oilpolymerization, for example.

[0044] In the case of the process of inverse suspension polymerizationand that of water-in-oil polymerization, the oil phase used comprisessaturated hydrocarbons, examples being hexane, heptane, cyclohexane, anddecalin, or aromatic hydrocarbons, such as benzene, toluene, xylene, andcumene. The ratio of oil phase to aqueous phase in the case of inversesuspension polymerization is, for example, from 10:1 to 1:10.

[0045] (Co)polymers having a low K value, of below 80, for example, areobtained if the (co)polymerization is conducted in the presence ofpolymerization regulators or in a solvent that regulates the(co)polymerization, examples being alcohols, such as methanol, ethanol,n- or iso-propanol, or ketones, such as acetone, ethyl methyl ketone,diethyl ketone or iso-butyl methyl ketone.

[0046] K values of low molecular weights and correspondingly low Kvalues are also obtained by means of the customary methods, i.e., use ofrelatively large amounts of polymerization initiator or ofpolymerization regulators, or combinations of said measures.

[0047] The molecular weight of the (co)polymers that can be used inaccordance with the invention is not restricted; however, it should notbe too high, so that the coating slip does not have too high aviscosity. Preference is given to (co)polymers having K values ofbetween 10 and 80, with K values between 30 and 70 being particularlypreferred.

[0048] In accordance with the invention, vinylformamide (co)polymers maybe used either in partially or fully cleaved form or else in uncleavedform. Preference is given to a degree of hydrolysis of between 0 and30%, with particular preference between 0 and 20%, and with veryparticular preference between 0 and 10%. The nature of the eliminationof the formyl group is not restricted, and elimination may be performed,for example, in the presence of acid or base; preference is given tocleavage in the presence of bases, such as sodium hydroxide, potassiumhydroxide, alkaline earth metal hydroxides, ammonia or amines, forexample. In this case, partial hydrolysis of, for example, a copolymercontaining (meth)acrylates and vinylformamides in copolymerized form maygive rise to amphoteric (co)polymers.

[0049] In a particularly simple way, however, cationic copolymers ofvinylformamide are obtained by cleaving homopolymers of vinylformamidehydrolytically to the desired degree of hydrolysis using defined amountsof acid or base, as described in EP-B1 071 050. Depending on the pH ofthe solution, the amino groups which are formed on the polymer chain aremore or less protonated and so give the polymer a more or less cationiccharacter.

[0050] If it is desired to eliminate the formyl group, this may be donein water.

[0051] The elimination of the formyl group in the hydrolysis takes placeat temperatures in the range from 20 to 200° C., preferably from 40 to180° C., in the presence or absence of acids or bases. The hydrolysis ispreferably conducted within the temperature range from 70 to 90° C.

[0052] For the acidic hydrolysis, from about 0.05 to 1.5 equivalents ofan acid, such as hydrochloric acid, hydrobromic acid, phosphoric acid orsulfuric acid, are required per formyl group equivalent in thepoly-N-vinylformamide. The pH for the acidic hydrolysis is in the rangefrom 2 to 0, preferably from 1 to 0. The hydrolysis proceeds withsubstantially greater rapidity than that of (co)polymers of otherN-vinylcarboxamides, such as of N-methyl-N-vinylformamide, for example,and may therefore be conducted under more gentle conditions, i.e., atlower temperatures and -without a large excess of acids.

[0053] Furthermore, the hydrolysis of the formyl groups of thepoly-N-vinylformamide may also be carried out in an alkali medium, inthe pH range from 11 to 14, for example. This pH is preferably set byadding sodium hydroxide or potassium hydroxide solution. It is, however,also possible to use ammonia, amines and/or alkaline earth metal bases.From 0.05 to 1.5, preferably from 0.4 to 1.0, equivalent(s) of a base is(are) used for the alkaline hydrolysis.

[0054] The cleavage may also be carried out at high temperatures, forexample, above 100° C., preferably from 120 to 180° C., with particularpreference from 140 to 160° C., in the presence of a solvent, e.g.,water, without acid or base. This is preferably done under conditionsabove the critical point, using supercritical water, for example.

[0055] In the course of the hydrolysis—i.e., the formyl group iseliminated from the poly-N-vinylformamide in water in the presence ofacids or bases—the byproduct comprises formic acid and/or salts offormic acid.

[0056] The resulting solutions may be used without further workup, orelse the hydrolysis and/or solvolysis products may be separated off.

[0057] For separation, the solutions obtained are treated using ionexchangers, for example. The residue separated from the hydrolysisproducts may then be incorporated into the coating slips.

[0058] The amount of vinylformamide (co)polymers added to the papercoating slip of the invention is guided by the amount of brightener inthe slip.

[0059] Normally, from 0.2 to 2 parts by weight of optical brightener per100 parts by weight of pigment are added to the coating slip. The amountof (co)polymer added to the coating slip is normally equal to from 1 to5 times the amount of the optical brightener, i.e., from 0.2 to 10 partsby weight, preferably from 0.5 to 8 parts by weight, and with particularpreference from 1 to 5 parts by weight.

[0060] The paper coating slips of the invention preferably comprise atleast one optical brightener.

[0061] The coating slips of the invention are processed completely inanalogy to processing of coating slips in accordance with the prior art,e.g., in accordance with “The Essential Guide to Aqueous Coating ofPaper and Board”, T. W. R. Dean (ed.), Published by the Paper IndustryTechnical Association (PITA), 1997 or “Ratgeber für die Verwendung vonBASF-Erzeugnissen in der Papier-und und Kartonstreicherei”, BASFAktiengesellschaft, D-6700 Ludwigshafen, Federal Republic of Germany, B376 d, 09.77.

[0062] Besides the additive of the invention, the paper coating slips ofthe invention further comprise at least one white pigment and at leastone binder.

[0063] The paper coating slips may further comprise other ingredientsknown to the skilled worker. Suitable examples include levelingassistants, pigment wetting aids, etc.

[0064] There is no restriction of the optical brighteners that may beused in connection with the coating slips of the invention. It ispossible to use the commercially customary stilbene derivativessubstituted by up to 6 sulfonic acid groups, an example beingBlankophor® PSG from Bayer AG, or derivatives thereof, or4,4′-distyrylbiphenyl derivatives.

[0065] The pigments that may be used in the coating slips of theinvention are likewise not subject to any restriction. For example, usemay be made of satin white (calcium sulfoaluminate), calcium carbonatein ground or precipitated form, barium sulfate in ground or precipitatedform, kaolin (clay), calcined clay, talc, silicates, chalk or coatingclay, or organic pigments, e.g., plastics in particle form.

[0066] Nor is there any restriction on the binders ((co)polymericbinders) that may be used in the coating slips of the invention. Forexample, casein, starch, soy protein, carboxymethylcellulose, alginateand/or polyvinyl alcohol or dispersions containing acrylic acid,acrylates, vinyl acetate and/or styrene in copolymerized form, e.g.,(co)polymers of acrylate/styrene, styrene/butadiene or vinyl acetate,may be used. The paper coating slips may further comprise, for example,dispersants. Suitable dispersants are polyanions, for example those ofpolyphosphoric acids or of polyacrylic acids (polysalts), which arenormally present in amounts of from 0.1 to 3% by weight, based on thepigment amount.

[0067] To prepare the paper coating slip, the ingredients are mixedconventionally, with the (co)polymer being used generally in the form ofa dispersion, suspension or solution.

[0068] The amount of water in the paper coating slip is usually adjustedto from 25 to 75% by weight, based on the overall paper coating slip(including water).

[0069] The paper coating slip may be applied by customary techniques tothe papers to be coated (cf. Ullmann's Encyclopädie der TechnischenChemie, 4th edition, Vol. 17, p. 603 ff).

[0070] If desired, a thickener may be added as well. Suitable thickenersinclude free-radically (co)polymerized (co)polymers and customaryorganic and inorganic thickeners such as hydroxymethylcellulose orbentonite.

[0071] In the majority of cases, the paper coating slips are aqueousslips. The water content may be adjusted in accordance with the desiredviscosity or flow properties.

[0072] To prepare the paper coating slip, the ingredients may be mixedin a known manner. The paper coating slips of the invention are suitablefor coating, for example, paper or board. The paper coating slip maythen be applied to the papers or board by conventional techniques.

[0073] The papers or boards coated with the paper coating slips of theinvention may be printed in customary processes, e.g., offset,letterpress or gravure printing processes.

[0074] The examples below are intended to illustrate the properties ofthe paper coating slips of the invention but without restricting theinvention to these particular coating slips.

[0075] In this specification, parts are by weight unless otherwisespecified.

EXAMPLE 1

[0076] A coating slip was prepared having the following composition:

[0077] 70 parts of calcium carbonate (Hydrocarb® 90, Plüss-Staufer AG)

[0078] 30 parts of kaolin (Amazon 88, Kaolin International)

[0079] 8 parts of styrene-butadiene latex (Styronal® 610, BASFAktiengesellschaft)

[0080] 0.5 part of optical brightener (Blankophor® PSG, Bayer AG)

[0081] and as cobinder:

[0082] 0.5 part of carboxymethylcellulose (CMC 7L2T, Hercules GmbH) or

[0083] 0.5 part of polyvinylformamide (PVFA) having a K value of 51 anda degree of hydrolysis as specified in table 1.

[0084] The coating slips, with a solids content of 68.1% by weight, wereapplied at a coat weight of 16 g/m² to chemical paper with a basisweight of 70 g/m² and the paper was then glazed.

[0085] The brightening of the paper was determined in accordance withDIN 53 145, part 2.

[0086] The CIE whiteness of the paper was measured in accordance withISO 2469.

[0087] When determining the dry pick resistance in accordance with theIGT method using the Lorilleux ink 3808 at 85 cm/s, the ink density wasdetermined using the Gretag densitometer.

[0088] When determining the wet pick resistance in accordance with usingthe test bench instrument using the Lorilleux ink 3804 at 35 cm/s, theink density was determined using the Gretag densitometer.

[0089] Additionally, the 75° print gloss of the papers was measured bythe Lehmann method.

[0090] The test results for the coated papers are given in table 1.TABLE 1 PVFA PVFA PVFA Degree of Degree of Degree of hydrolysishydrolysis hydrolysis Cobinder CMC 7L2T 0% 3% 8.5% Whiteness % 91.0 92.993.9 94.1 R 457 with UV Whiteness % 85.9 86.4 87.0 87.1 R 457 without UVBrightening % 5.1 6.5 6.9 7.0 Whiteness % 102.3 109.2 111.4 111.3 CIEIGT dry 0.53 0.94 1.25 1.25 (85 cm/s) Test bench wet 0.70 1.71 1.81 1.55(35 cm/s) Print gloss % 69.5 71.8 72.1 76.6

[0091] The coating slips of the invention which comprise apolyvinylformamide (PVFA) experience greater optical brightening andpossess a higher CIE whiteness than the coating slips which, inaccordance with the prior art, comprise carboxymethylcellulose ascobinder. It is also evident that polyvinylformamides having undergone acertain degree of hydrolysis and therefore being cationic providegreater brightening and CIE whiteness than nonionic PVFA. Moreover, itcan be seen that the coating compositions that include apolyvinylformamide have much better properties in terms of both dry andwet pick resistance than those comprising CMC as cobinder. Finally, thecoating slips of the invention also produce a markedly higher printgloss.

EXAMPLE 2

[0092] A coating slip was prepared having the following composition:

[0093] 70 parts of calcium carbonate (Hydrocarb 90, Plüss-Staufer AG)

[0094] 30 parts of kaolin (Amazon 88, Kaolin International)

[0095] 8 parts of styrene-butadiene latex (Styronal® 615, BASFAktiengesellschaft)

[0096] 0.5 part of optical brightener (Blankophor PSG, Bayer AG)

[0097] The cobinders used were as follows:

[0098] 0.5 part of carboxymethylcellulose (CMC 7L2T, Hercules GmbH) or

[0099] 0.5 part of polyvinyl alcohol (Mowiol® 6-98, ClariantAktiengesellschaft) or

[0100] 0.5 and

[0101] 1.0 part of polyvinylformamide (PVFA) having a K value of 45 anda degree of hydrolysis of 5%.

[0102] The coating slips were processed as described in example 1.

[0103] The brightening and whiteness of the glazed papers, and the printgloss, were determined as described in example 1.

[0104] The test results are set out in table 2. TABLE 2 Polyvinylalcohol Cationic PVFA CMC Mowiol Degree of 7L2T 6-98 hydrolysis 5%Cobinder none 0.5% 0.5% 0.5% 1.0% Whiteness % 89.8 91.2 93.0 94.3 95.0 R457 with UV Whiteness % 85.0 85.8 86.1 87.0 86.8 R 457 without UVBrightening % 4.8 5.4 6.9 7.3 8.2 Whiteness % 99.3 103.5 111.0 113.1116.6 CIE Print gloss % 72.6 72.2 70.2 73.8 75.5

[0105] Table 2 reveals that the coating slips of the inventioncontaining polyvinylformamide produce a higher whiteness and a greaterprint gloss than the coating slips that contain other cobinders.

EXAMPLE 3

[0106] A coating slip was prepared having the following composition:

[0107] 70 parts of calcium carbonate (Hydrocarb 90, Plüss-Staufer AG)

[0108] 30 parts of kaolin (Amazon 88, Kaolin International)

[0109] 8 parts of styrene-butadiene latex (Styronal® PR 8736, BASFAktiengesellschaft)

[0110] 0.5 part of optical brightener (Blankophor PSG, Bayer AG)

[0111] The cobinders used were as follows:

[0112] 1.0 part of carboxymethylcellulose (CMC 7L2T, Hercules GmbH) or

[0113] 1.0 part of acrylic-based copolymer (Acrosol® C 50 L, BASFAktiengesellschaft) or

[0114] 1.0 part of polyvinylformamides (PVFA) having a K value of 50 anda degrees of hydrolysis of 1% and 5%.

[0115] The coating slips were processed as described in example 1.

[0116] The brightening and whiteness of the glazed papers, and their dryand wet pick resistances, were determined as described in example 1.

[0117] The test results are set out in table 3. TABLE 3 PVFA PVFA Degreeof Degree of Acrosol C hydrolysis hydrolysis Cobinder CMC 7L2T 50 L 1%5% Whiteness % 91.9 92.7 93.4 94.3 R 457 with UV Whiteness % 86.2 86.386.1 86.5 R 457 without UV Brightening % 5.7 6.4 7.3 7.8 Whiteness %105.3 107.8 111.6 113.8 CIE Test bench wet 0.55 0.52 1.44 1.21 (130cm/s)

[0118] The results set out in table 3 confirm that using the coatingslips of the invention prepared with the additives described gives riseto papers having higher whiteness and greater strength than when usingcoating slips comprising prior art cobinders.

EXAMPLE 4

[0119] A coating slip was prepared having the following composition:

[0120] 70 parts of calcium carbonate (Hydrocarb 90, Plüss-Staufer AG)

[0121] 30 parts of kaolin (Amazon 88, Kaolin International)

[0122] 10 parts of styrene-acrylate latex (Acronal® S 360 D, BASFAktiengesellschaft)

[0123] 0.5 part of carboxymethylcellulose (CMC 7L2T, Hercules GmbH) ascobinder for all formulas

[0124] 0.5 part of optical brightener (Blankophor PSG, Bayer AG)

[0125] As an additional cobinder the following were used:

[0126] 0.5 part of carboxymethylcellulose (CMC 7L2T, Hercules GmbH) or

[0127] 0.5 part of polyvinylformamide (PVFA) having a K value of 69 anda degree of hydrolysis of 1%.

[0128] The coating slips were processed as described in example 1.

[0129] The brightening and whiteness of the glazed papers and their dryand wet pick resistances were determined as described in example 1.

[0130] The test results are reproduced in table 4. TABLE 4 PVFA K value69 Degree of hydrolysis Additional cobinder CMC 7L2T 1% Whiteness R 457% 94.0 95.8 with UV Whiteness R 457 % 87.4 88.5 without UV Brightening %6.6 7.3 Whiteness CIE % 110.7 111.6 IGT dry 1.35 1.80 (170 cm/s) Testbench wet 0.78 1.28 (130 cm/s)

[0131] The results in table 4 show that the coating slips of theinvention produce an improvement even when they already contain aconventional cobinder, the addition of the novel additive making thequality of the coated paper much higher than when the same amount of thecobinder already present in the slip is added.

EXAMPLE 5:

[0132] A coating slip was prepared having the following composition: 70parts of calcium carbonate (Hydrocarb 90, Plüss-Staufer AG) 30 parts ofkaolin (Amazon 88, Kaolin International) 10-12 parts (see table 5) ofstyrene-butadiene latex (Styronal LD 615, BASF Aktiengesellschaft) 0.5part of optical brightener (Blankophor PSG, Bayer AG)

[0133] Sterocoll® FD as indicated in table 5.

[0134] The cobinders used were as follows:

[0135] 1.0 part of oxidatively degraded starch (Emox® TSC,Emsland-Stärke GmbH) or

[0136] 2.0 parts of oxidatively degraded starch (Emox TSC,Emsland-Stärke GmbH) or

[0137] 0.5 part of polyvinylformamide (PVFA) with a K value of 69 and adegree of hydrolysis (DH) of 1% and 5% respectively.

[0138] The coating slips were processed as described in example 1.

[0139] The brightening and whiteness of the glazed papers and their wetpick resistances were determined as described in example 1.

[0140] The blister resistance was determined by immersing the papers,coated on both sides, in hot oil (240° C.). The blistering was assessedwith a rating from 1 (no blisters) to 6 (very many blisters).

[0141] The test results are set out in table 5. TABLE 5 Parts of 12 1110 11 11 Styronal LD 615 Parts of 0.30 0.25 0.2 0.5 0.5 Sterocoll FDCobinder — Oxid. Oxid. PVFA PVFA (parts) starch starch DH 1% DH 5% (1.0)(2.0) (0.5) (0.5) Whiteness R % 92.2 92.9 92.6 94.5 93.4 457 with UVBrightening % 4.5 4.8 4.7 6.2 6.4 Whiteness % 100.8 103.2 103.4 108.6109.4 CIE Test bench wet 1.11 1.32 0.93 1.69 1.21 (135 cm/s) Blisteringat 6 3 5 1 2 240° C.

[0142] Table 5 reveals that although degraded starches likewise increasethe whiteness of the paper, they fail to achieve the effectiveness inthis respect of the much smaller amounts of polyvinylformamides. It isalso evident that the papers produced using polyvinylformamides ascobinders combine an equivalent or higher wet pick resistance with asubstantially reduced blistering tendency in comparison with the papersproduced using starch.

EXAMPLE 6

[0143] A coating slip was prepared having the following composition:

[0144] 70 parts of calcium carbonate (Hydrocarb 90, Plüss-Staufer AG)

[0145] 30 parts of kaolin (Amazon 88, Kaolin International)

[0146] 8 parts of styrene-butadiene latex (Styronal D 610, BASFAktiengesellschaft)

[0147] 0.5 part of optical brightener (Blankophor PSG, Bayer AG)

[0148] The following cobinders were used:

[0149] 0.5 part of carboxymethylcellulose (CMC 7L2T, Hercules GmbH) or

[0150] 0.5 part of a copolymer of vinylformamide and acrylic acid(VFA/AA) in a ratio of 80:20 with a K value of 38 or

[0151] 0.5 part of a copolymer of vinylformamide and acrylic acid(VFA/AA) in a ratio of 90:10 with a K value of 43.

[0152] The coating slips were processed as described in example 1.

[0153] The brightening and whiteness of the glazed papers weredetermined as described in example 1.

[0154] The test results are recorded in table 6. TABLE 6 80:20 VFA/AA90:10 VFA/AA Cobinder CMC 7L2T copolymer copolymer Whiteness R 457 %91.3 91.6 92.4 with UV Whiteness R 457 % 85.9 86.1 86.2 without UVBrightening % 5.1 5.5 6.2 Whiteness CIE % 102.3 104.6 106.7

[0155] The tests show that even the addition of anionicpolyvinylformamides produces coating slips which give rise to paperswith greater whiteness than coating slips containing prior artcobinders.

EXAMPLE 7

[0156] A coating slip was prepared having the following composition:

[0157] 70 parts of calcium carbonate (Omyalite® 90, Plüss-Staufer AG)

[0158] 30 parts of kaolin (Amazon 88, Kaolin International)

[0159] 10 parts of styrene-acrylate latex (Acronal S 305 D, BASFAktiengesellschaft)

[0160] 0.5 part of optical brightener (Blankophor PSG, Bayer AG)

[0161] The cobinders used were as follows:

[0162] 2 parts of polyvinyl alcohol (Polyviol® LL 603, Wacker-ChemieGmbH) or

[0163] 3.5 parts of oxidatively degraded starch (Emox TSC,Emsland-Stärke GmbH) or

[0164] 2 parts of polyvinylformamide (PVFA) having a K value of 32 anddegrees of hydrolysis of 0%, 1% and 5% respectively.

[0165] The coating slips were processed as described in example 1.

[0166] The brightening and whiteness of the glazed papers and their dryand wet pick resistances, were determined as described in example 1.

[0167] The test results are set out in table 7. TABLE 7 PVFA PVFA PVFAdegree of Degree of Degree of Polyvinyl Oxid. hydrolysis hydrolysishydrolysis Cobinder alcohol Starch 0% 1% 5% Whiteness % 94.0 92.2 95.296.0 95.6 R 457 with UV Whiteness % 86.4 86.1 86.8 87.4 87.2 R 457without UV Brighten- % 7.6 6.1 8.4 8.6 8.4 ing

[0168] Table 7 shows that even the addition of nonionic and cationicpolyvinylformamides having a low molecular weight (K value) to coatingslips produces papers having greater optical brightening than can beachieved by adding prior art cobinders.

1. The use of an addition polymer or copolymer containingN-vinylformamide in (co)polymerized form for activating opticalbrighteners in paper coating slips.
 2. The use as claimed in claim 1,wherein further to the N-vinylformamide polymer or copolymer the papercoating slip comprises an optical brightener, at least one white pigmentand at least one binder.
 3. The use as claimed in claim 1 or 2, whereinthe copolymer additionally contains at least one water-soluble cationicmonomer in copolymerized form.
 4. The use as claimed in any of claims 1to 3, wherein the copolymer further contains acrylic acid or methacrylicacid and/or their salts or mixtures thereof in copolymerized form. 5.The use as claimed in any of claims 1 to 4, wherein the copolymerfurther contains N-vinylpyrrolidone in copolymerized form.
 6. The use asclaimed in any of claims 1 to 5, wherein the copolymer further containsat least one additional water-soluble vinyl monomer in copolymerizedform.
 7. The use as claimed in any of claims 1 to 6, wherein theN-vinylformamide present in (co)polymerized form in the addition polymeror copolymer has undergone partial cleavage.
 8. A paper coating slipcomprising at least one binder, at least one white pigment, an opticalbrightener, and an addition polymer or copolymer as set forth in any ofclaims 1 to
 6. 9. A paper coating slip as claimed in claim 8, whereinthe copolymer is a cationic copolymer.
 10. A paper coating slip asclaimed in claim 8 or 9, wherein the addition polymer or copolymer iscomposed of: from 1 to 100 parts by weight of n-vinylformamide from 0 to10 parts by weight of water-soluble cationic monomer from 0 to 30 partsby weight of acrylic acid, methacrylic acid, salts thereof or mixturesof these from 0 to 90 parts by weight of an additional water-solublevinyl monomer.
 11. A paper coating slip as claimed in any of claims 8 to10, wherein the addition polymer or copolymer is present in amounts offrom 0.2 to 10 parts by weight per 100 parts by weight of pigment. 12.Paper coated with a slip as claimed in any of claims 8 to
 11. 13. Theuse of paper as claimed in claim 12 in a printing process.